Films formed of plastic resins have superior characteristics and are used in many industries. Typical films are exemplified by polyester films, aramid films, olefin films, polypropylene films, PPS (polyphenylene sulfide) films, and PET (polyethylene terephthalate) films. Besides, there are also known films made of PE (polyethylene), POM (polyoxymethylene or acetal resin), PEEK (polyether ether ketone), ABS resin (acrylonitrile, butadiene and styrene copolymerization synthetic resin), PA (polyamide), PFA (ethylene tetrafluoride-perfluoroalkoxyethylene copolymer), PI (polyimide), PVD (polyvinyl dichloride), etc.
In the fields related to smartphones, tablets, liquid-crystal displays, solar panels, automobiles, etc., there is expanding the application of high-function films as indispensable components. The films are made to have high functions not only by film material, but also by technologies such as high functionality of the film layer structure (bulk control), multilayer lamination, surface control, etc. Furthermore, flexible devices that function even on products subjected to deformation are produced by forming electronic devices, wirings, etc. on these films. Thus, the possibility of their application is expanding, such as wearable computer, digital signage capable of being attached to and installed in various places, etc.
In the film multilayer lamination and the film surface control, there is conducted the film surface modification such as in Patent Publication 1. In general, the surface modification is to modify a hydrophobic (water-repellent) film surface to have hydrophilicity, for the purpose of barrier property improvement, durability improvement, wettability improvement (adhesion improvement), etc. That is, the film surface is modified by destroying the molecular structure of the film material surface with a certain means and adding hydrophilic OH groups, O groups, etc.
As specific film surface modification means, there are (1) a physical modification method such as corona discharge, plasma treatment, sputtering treatment, etc., (2) a modification method by UV light irradiation or electron beam irradiation, etc., (3) a modification method by a reactive gas such as ozone, (4) a wet method by a liquid chemical, ozone water, etc., and the like.
It is considered that the modification method (2) by UV light irradiation or electron beam irradiation, etc. is not suitable as a method for modifying only the film surface in view of the film having a property to easily transmit these UV lights and electron beams.
Furthermore, the modification method (3) by a reactive gas has a risk that a sufficient surface modification effect is not obtained. For example, even in ozone gas as a typical reactive gas, there is a risk that oxygen radicals as the reaction active species contributing to the modification reaction cannot sufficiently be supplied to the film surface. Thus, in order to increase supply of oxygen radicals, it is also conducted to forcibly decompose ozone gas by irradiating ozone gas with UV light (e.g., Patent Publication 2). However, oxygen radicals generated by UV light irradiation are oxygen radicals in excited state and extremely high in reactivity. Therefore, its control is difficult. That is, in case that ozone gas is irradiated with UV light, the modification effect is higher as compared with the surface modification with only ozone gas, but it is difficult to supply oxygen radicals to the film surface. Therefore, there is a risk that a sufficient modification effect cannot be obtained.
Furthermore, in the wet modification method (4), there is a risk that its application field is limited by the reasons such as the necessity to have a step of drying the film after the treatment, the necessity to have a post-treatment of a liquid chemical used in the modification treatment and to have a treatment facility, etc.
By such reasons, the physical modification method (1) as being highest in modification effect is the most widely used.
However, in corona discharge, plasma treatment, etc., which are frequently used as the physical modification method (1), there is a risk that a fine modification control, such as uniformity of the modification effect, becomes difficult. This is because the discharge phenomena in corona discharge or plasma treatment is caused by dielectric breakdown of a gas between the electrodes to result in the modification effect being affected by the electrode shape or the electrode surface condition. Furthermore, there is a considerable risk that a high temperature caused by heat generation accompanied with discharge considerably damages the film that becomes a substrate. Moreover, since the electrode surface deteriorates over time by discharge, there is a risk that a constant modification effect cannot be obtained over a long period of time.
Furthermore, in a method such as the physical modification method (1), there is a risk that the modification effect is lost as time passes. For example, it is known that, in a film having a small contact angle of a water drop by a hydrophilic treatment, the contact angle of a water drop returns to a state close to that prior to the treatment in several days. Therefore, it is believed that a long-term storage of the film is difficult. Thus, in some cases, the hydrophilic treatment is conducted two times immediately after the film production and prior to the film use in order to maintain hydrophilicity of the film.